1. Field of the Invention
This invention relates to apparatus and processes for the indirect measurement and control of fluid flow rate and the measurement of viscosity-related properties of fluids flowing in systems having a variable speed pumping capability, and for monitoring the mechanical performance of the pump.
The measurement and control of a fluid flow rate can be an important facet of many industrial processes and applications. For example, in the chemical industries wherein one or more fluid materials are introduced into a reaction chamber, accurate control over the flow rates of the separate liquids ensures that proper amounts of the separate reactants will be metered through the reaction chamber to provide optimum product yields and economically efficient operation.
The need for monitoring and control of fluid flow rates is especially important in continuous process applications. In such applications, little or no opportunity exists for testing the reactant mix and subsequently adding a measured amount of one or more reactants to achieve the desired balance. Typical of such non-intermittent applications are certain wood-pulp processing operations which require the continuous flow of a fluid component but at varying flow rates which are determined by other process considerations.
There are two general approaches presently in use for the control of fluid flow rates: (1) opening or closing a control valve in the fluid system while maintaining the pump at constant speed; and (2) increasing or decreasing the speed of the pump while maintaining any control valves in the full-open position. Until the invention of the apparatus and processes to be set forth in more detail hereafter, the pump speed flow control has not been the preferred mode due, in part, to the need for inline flow measurement apparatus to provide a flow signal for control and to the higher initial cost for the variable speed pump drive capability.
There are, however, several distinct benefits and advantages to be gained from using the pump speed flow control mode over the control valve flow control system. First, and most important, the power needed to operate the pump in a pump speed control system can be substantially less than the power expended in a comparable system having control valve control. In either system, the pump will be sized to accomodate the maximum expected flow rate and so the peak power requirements of both systems will be the same. However, the pump in the system controlled by a control valve will always be operating at the maximum pump speed while the pump in the pump speed controlled system will operate at maximum speed (and hence capacity) only when maximum flow is actually required.
The power requirements of a centrifugal pump-driven fluid system vary as the product of the pressure rise and the flow rate. Hence, it is to be expected that less power is expended in a pump speed controlled system on a time-average basis than in a comparable fluid system having control valve flow control.
Throughout this specification and also in the claims, the terms "system flow rate," "pump flow rate," and "flow rate" will be used interchangeably. Since the dynamic response of the pump and the piping system will not be considered, the pump and system being always operated in a quasi-steady state condition of flow equilibrium, the instantaneous values of the above-mentioned terms will be equal. Similarly the terms "pump pressure rise" and "pressure rise" will be used interchangeably.
In connection with the use of the latter terms during the course of the subsequent discussion, it must be remembered that the term "pump pressure rise" usually given in units of psi should not be equated to the term "pump head" or "head" in widespread use in the fluid flow art which typically is given in units of feet of the pumped liquid at a particular temperature. The temperature dependence of the "head" variable occurs via the density dependence, and caution must be exercised in converting from measured values of one variable to the other for situations in which significant swings in temperature occur.
In practice, it is extremely difficult, if not impossible, to accurately measure the "head" variable directly, such as by the use of manometers the liquid in which can undergo significant temperature changes. The use of the terms "pressure rise" and "pump pressure rise" throughout the remainder of this specification is intended to reflect the static pressure difference variable that is independent of density.
Another advantage to the pump speed flow control mode involves the pumping efficiency of the particular centrifugal pump employed. Typically, centrifugal pumps are designed to exhibit maximum efficiency at the design flow rate. It is a characteristic of many centrifugal pumps that the decrease in efficiency is less in going from the design flow rate (and pump speed) to a lower flow rate by reducing pump speed than the decrease in pump efficiency caused by an increase in the piping system resistance needed to achieve the same final flow rate. Hence, an additional power savings over and above that described previously can be gained from the use of pump speed flow control.
Other advantages of the use of the pump speed flow control mode include the possible elimination entirely of the control valve, together with the vibration and noise incident to valve throttling, and the attendant maintenance costs. The use of a pump speed flow control mode will also extend the life of pump components, including shaft, sealing glands, bearings and gears, all of which are affected by the high speeds and high discharge pressures that occur at high throttling conditions. The possibly higher capital costs for the use of a more complex pump driver for the pump speed flow control mode can be more than off-set with the above-detailed savings in power expenditure and equipment down-time.
The above-described phenomenon and considerations are well known in the fluid flow art but have been presented herein in order to clarify and accentuate the importance of the invention to be described henceforth.
2. Brief Description of the Prior Art
The U.S. Pat. No. 2,734,458 to Hayes and the U.S. Pat. No. 3,024,730 to Towle, show typical automatic flow control arrangements using pump speed control. The automatic control systems used in these references generally include (1) apparatus to measure water level in a sump or reservoir; (2) apparatus for determining the error between the desired value of the water level and the actual measured value; and, (3) control apparatus that responds to the measured error by adjusting pump speed in the direction which will cause the error to vanish. The Hayes and Towle references, through a system of liquid-filled variable resistors or cam-and-link operated potentiometers, convert the measured liquid level to a corresponding change in the voltage across the secondary windings of the pump motor to control the speed of the motor. Hayes and Towle both recognized the inherent cost savings that accrue by the use of pump speed control in their respective systems.
One inherent shortcoming of flow control systems such as shown in Hayes and Towle, wherein the water level in a sump or reservoir system component is used as the monitor to flow-related variable, is that many industrial processes employ a closed system having no such accumulator elements where a fluid level could be identified and measured. Also, and more importantly, the measurement of a fluid level yields an indication only of the integrated flow rate over a period of time. Such a system can only be used to control at best, a time-average system flow rate. Many industrial processes require an indication and control of the instantaneous flow rate for proper operation.
Direct measurement of the instantaneous flow rate, however, ordinarily requires the introduction of sophisticated measuring apparatus to the fluid system. Such flow measuring devices as venturis or turbine rotor flow meters when installed in the system will add flow resistances that can materially degrade the performance of the overall system. These devices also will increase the initial cost of a fluid system and contribute to the operating cost by requiring frequent periodic maintenance. It is apparent, then, that apparatus and a process for obtaining pump speed flow control which utilize a measurement of the instantaneous flow rate but which does not necessitate the use of the conventional flow measuring devices would be highly desirable.
The use of easily measurable flow-related variables to provide indirect measurement and control of pump pressure rise is known in the art. Such variables typically are associated with the operating characteristics of the pump or pump drive motor. U.S. Pat. No. 3,563,672 to Bergstrom discloses a system for limiting peak pump pressure rise through pump speed control by monitoring the armature current in the pump motor, a variable which also can be easily measured without disturbing the actual fluid system. The use of this associated pressure-related variable, the armature current, by the automatic control system is made possible by the linear interdependence of the current and pressure rise variables for the positive displacement pump used in Bergstrom, as is depicted in FIG. 3 of that reference.
A highly desirable alternative to the use of conventional flow measuring devices to provide an indication of the instantaneous flow rate would be apparatus and a process for measuring two or more flow-related variables associated with the pump and pump drive apparatus such as, but not limited to, pump pressure rise, pump speed, or pumping power and using the measured values to determine the instantaneous value of the system flow rate. Theoretical relationships can be determined which give flow rate as a function of any two of such flow-related variables for systems wherein fluid temperature and viscosity remain relatively constant. Although the functional relations are complex, various mathematical techniques are available for the rapid solution of the equations or other mathematical relationships, including the use of computers. Hence, given the instantaneous values of the two flow-related variables, the equations relating flow rate to these variables could be used to solve for the corresponding value of the instantaneous flow rate.
Although it is known in the art that the instantaneous flow rate of a centrifugal pump-driven fluid system can be determined by measuring any two of certain flow-related parameters such as pump pressure rise, pump speed, and pump power it has been heretofore impossible to utilize these theoretical relationships in an automatic control system because of the inherent non-linearities in the governing equations. For instance, headflow curves for a centrifugal pump typically exhibit a quadratic relationship. This form of variable dependency does not lend itself to conventional control systems, either analog or digital, which require linear dependency.
The present invention solves the aforementioned problems by providing apparatus and process which permit a fluid system flow rate to be automatically and continuously measured and controlled by varying pump speed in response to a flow signal derived from measured values of certain flow-related variables. The apparatus and process of this invention enable all the benefits and advantages of a pump speed flow control mode hereinbefore described to be achieved while eliminating or substantially reducing the disadvantages of directly monitoring instantaneous flow rate by the use of conventional flow rate measuring apparatus and processes.
Additionally, the apparatus and processes of the present invention allow monitoring of the mechanical performance of the pump without the need to measure flow directly, to provide an indication of unacceptable degradations in pumping capability such as caused by a worn or broken impeller.
Also, the apparatus and processes of the invention to be disclosed hereinafter enable certain properties related to the viscosity of the pumped fluid to be determined automatically and continuously without the need to employ conventional measuring devices and techniques.
Such properties include the kinematic viscosity of ordinary liquids and the "consistency" of the liquid-fiber slurries commonly found in wood pulp processing applications, as that term is used in the publication entitled Cameron Hydraulic Data, G. V. Shaw and A. W. Loomis, Ingersoll-Rand Co., 11th ed., c 1942. The "consistency" of a fiber-water slurry is roughly defined as the weight percent of fiber in the slurry but can also be affected by the fiber type, degree of air entrainment, and other parameters.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.